1. Field of the Invention
The present invention relates to an original-document illumination apparatus used for digital copiers and image scanners. The invention, in particular, relates to an original-document illumination apparatus which employs a light emitting diode as a light source. The invention also relates to a color-original-document reading apparatus and an image reading apparatus which employ the original-document illumination apparatus. The invention furthermore relates to an image forming apparatus which employs the color-original-document reading apparatus and the image reading apparatus.
2. Description of Related Art
Development of light emitting diodes (LEDs) has been advanced recently, and LEDs having high brightness have been dramatically requested. LEDs, in general terms, have such advantages as longer service life, higher efficiency, higher resistance to impact, and their capability of emitting monochromatic lights. For this reason, application of LEDs to a variety of lightening purposes is expected.
Among other things, LEDs are actually used in an original-document illumination apparatus of an image reading apparatus, such as digital copiers and image scanners.
FIG. 7 schematically shows an image forming apparatus 10, which has such an image reading apparatus as described above.
The image forming apparatus 10 shown in the example of FIG. 7 includes an image forming unit 100 and an image reading apparatus which is an image reading unit 200.
The image forming unit 100 includes a drum-shaped latent image carrier 111. The latent image carrier 111 is a photoconductive photo receptor. The uniform electrostatic charging of and the laser scanning of the latent image carrier 111 form an electrostatic latent image on the surface of the latent image carrier 111. The electrostatic latent image formed on the surface of the latent image carrier 111 is turned into a visible toner image. A roller charging device 112 as electrostatic charging means, a development apparatus 113, a transfer roller 114, and a cleaning apparatus 115 are disposed around the latent image carrier 111. FIG. 7 shows an example in which the roller charging device 112 is used as electrostatic charging means, but a corona charger may replace the roller charging device 112 as the electrostatic charging means. In addition, a laser scanning apparatus 117 is disposed around the latent image carrier 111. The laser scanning apparatus 117 receive data on an original-document that an external apparatus such as an image reading unit 200 sends, and scans the original document data by a laser beam (LB). The laser scanning apparatus 117 carries out “exposure by optical writing” between the roller charging device 112 and the development apparatus 113.
At image formation, the latent image carrier 111 which is a photoconductive photoreceptor uniformly rotates clockwise, and thus the surface of the latent image carrier 111 is electrostatically charged with uniformity by the roller charging device 112. In addition, the surface of the latent image carrier 111 is subjected to an exposure by optical writing with the laser beam LB of the laser scanning apparatus 117. As a result, an electrostatic latent image is formed on the surface of the latent image carrier 111. The electrostatic latent image thus formed is what is termed as a negative latent image. The image portion formed on the surface of the latent image carrier 111 is exposed to the laser beam LB. Transfer paper P is stored in a cassette 118, which is, in turn, is attachable to and detachable from the main body of the image forming unit 100. Now, assuming that the cassette 118 is attached to the image forming unit 100 as shown in FIG. 7, the upper most sheet of a plurality of sheets of the transfer paper P stacked in the cassette 118 is fed by a feed roller 120. The sheet of the transfer paper P thus fed is sent to the transfer unit while the front end portion of the sheet is held by a pair of register rollers 119. The pair of the register rollers 119 are configured to send the sheet of the transfer paper P to the transfer unit so as to be in time for the arrival of the toner image on the toner carrier 111 at the transfer position. The sheet of the transfer paper P thus sent to the transfer unit and the toner image are laid over each other in the transfer unit. The toner image that is laid over transfer paper P, with the operation of the transfer roller 114, is electrostatically transferred to the sheet of the transfer paper P. Once the toner image is transferred to the sheet of the transfer paper P, a cleaning apparatus 115 removes the toner and the paper dusts remaining on the surface of the latent image carrier 111. The sheet of transfer paper P on which the toner image is transferred is fed to a fixing unit 116. Feeding the sheet of the transfer paper P to the fixing unit 116 makes the toner image fixed to the transfer paper P. Thereafter, the sheet of the transfer paper P passes through a feeder path 121, and is then discharged out to a tray 123 by a pair of discharge rollers 122.
The image reading unit 200 includes a contact glass 201 on which an original-document 202 is placed, and a first carriage 203 that is disposed below the contact glass 201. An illumination member (not shown) is mounted on the first carriage 203 to illuminate original-document 202 on the contact glass 201. The light reflected from the original-document 202 is then reflected at the surface of a first mirror 203a that is mounted on the first carriage 203. Thereafter, the light is reflected on, a first and a second mirrors 204a and 204b of a second carriage 204, and then is guided to a reducing, image-forming lens 206. With the guided reflected light, the reducing, image-forming lens 205 forms an image on a line sensor 206.
When the original-document is read along the longitudinal direction thereof, the first carriage 203 is moved to the right-hand side of FIG. 7 at a speed V, and, concurrently, the second carriage 204 is moved to the right-hand side at half the speed of the first carriage 203, that is, at ½ V The entire original-document can be read in this way.
Despite the above-mentioned excellent properties, a single LED cannot emit light that is luminous enough to be used in an original-document illumination apparatus 11 (see FIG. 6) of an image reading apparatus 200. Accordingly, LEDs can be used in such apparatuses as a lower-speed reading apparatus and a compact-type reading apparatus, but cannot be used in such apparatuses as a higher-speed reading apparatuses and a large-type apparatus. Consequently, mainly, cold cathode fluorescent lamps are used in higher-speed reading apparatuses and larger apparatuses.
The above-mentioned problem of LEDs is generally dealt with by use of many LEDs arranged into an array to obtain increased amount of light of the original-document illumination apparatus 11 as a whole. In this case, however, the light of each LED spreads so widely as to lessen the efficiency and to contradict the promotion of energy saving products. To counteract such problems in efficiency and energy consumption, round type LEDs, which suffers less, among a plurality of types of LEDs, from the spread of their light, may be used in original-document illumination apparatus 11 of the image reading apparatus 200. The use of round type LEDs may possibly enhance efficiency. However, the light emitted from the round type LED is irradiated in a direction at a narrow angle, so that an uneven distribution of illuminance may possibly occur in the main-scanning direction.
In an original-document illumination apparatus that has been proposed thus far, an LED array, which includes a plurality of LEDs, and a lengthy lens are combined to accomplish more efficient use of light. For example, Japanese Patent Application Laid-open Publication No. H11-232912 and Japanese Patent No. 272660 disclose such an original-document illumination apparatus. In these apparatuses, the more efficient use of light is generally pursued by making the light from LEDs converge on a cross section along the sub-scanning direction of the LEDs. Such a method, however, has a problem. As shown in a drawing of Japanese Patent Application Laid-open Publication No. H111-232912, the center portion of the convergent light is bright, but a rapid drop in brightness is observed at a position away from the center. In this method, among the light emitted from the LEDs to the sub-scanning-direction cross section, most of the light emitted at an oblique angle to the optical axis is not used as an illuminating light, that is, most of the light is wasted. Therefore, an uneven distribution of brightness occurs in the main-scanning direction unless many LEDs are arranged.
The applicants of the present invention proposed, in a previous application, that is, Japanese Patent No. 3659770, a solution to the problem of the uneven distribution of brightness in the main-scanning direction, but no mention was made of the way how to condense light in the sub-scanning direction. In addition, the applicants of the present invention proposed, in another previous application, that is, Japanese Patent Application Laid-open Publication No. 2004-361425, a structure including, as an optical element, an optical guide that has a light-incoming plane arranged near the light-flux outgoing plane of the point light source, and also has a light-outgoing plane facing the reading area. According to the structure of this disclosure, the targeted distribution of brightness can favorably be obtained. The structure of this disclosure, however, needs reflectors in addition to the optical guide. The resultant complex structure brings about an increase in manufacturing costs.
Then, the applicants of the present invention proposed, in Japanese Patent Application Laid-open Publication No. 2005-278132, an original-document illumination apparatus that can solve all these problems. The original-document illumination apparatus disclosed therein is equipped with a light-source unit and a lengthy lens. The light-source unit has a plurality of rows each of which includes a plurality of LEDs with a certain distribution of luminous intensity. The lengthy lens makes the light emitted from each LED converge not at the position of the original-document within a cross section in the sub-scanning direction but at a position shifted, in the main-scanning direction, from the original-document position by a predetermined distance. The convergence of light in the sub-scanning direction is thus enhanced, so that a higher numerical aperture of the lengthy lens can be accomplished. In addition, the loss of light that may possibly be caused by light diffusion in the main-scanning direction is reduced. Consequently, even a smaller number of LEDs can make uneven distribution of brightness in the main-scanning direction less likely to occur.
The invention disclosed in Japanese Patent Application Laid-open Publication No. 2005-278132 has the above-mentioned advantages, but also has some drawbacks. Assume that the acute convergence of the light at a position on the area of the original-document is combined with a misalignment of illuminating position caused, for example, by an incorrect angle with which the lengthy lens is attached. What may possibly happen in this case is a big change in light quantity that reaches a light detector of a line sensor. This may possibly affect, to a great extent, the image to be formed by the apparatus. Incidentally, the relationship between the brightness or illuminance and the reading position when the illuminance on the original-document area is read along the main-scanning direction can be expressed as an illuminance-distribution curve. For the reason described above, a preferable original-document illumination apparatus for a digital copier or in an image scanner should have an illuminance-distribution curve which is wide, to some extent, in the main-scanning direction, and in which even a deviation of the illuminating center from the reading part will not cause a difference in illuminance within the reading area. To this end, the illuminance-distribution curve should have a flat portion near the point of the maximum illuminance, and the flat portion should have a width larger than a value made by adding a width needed for reading an image and a range of fluctuation caused by mechanical error and the like (for example, a preferable width is 1 mm or larger on each side). In addition, the flat portion should have a section where little unevenness occurs in the distribution of illuminance, that is, a section where the illuminance is substantially constant.